Blowing snow and sea ice surfaces as a source of polar sea salt aerosol (BLOWSEA)

Small particles (known as aerosol) in the atmosphere play several critical roles. They affect the transmission of sunlight to the underlying surface; they affect the formation of clouds, and they host and enhance important chemical reactions. When they are deposited on ice they leave a record of past conditions that can be accessed by drilling ice cores. The most significant aerosol component over marine areas is sea salt aerosol. Over most of the world's oceans this is created by bubble bursting in sea spray. However there is strong evidence that another source of sea salt aerosol is important in the polar regions, and that this ultimately derives from the surface of sea ice. The existence of this source forms the basis for a proposed method using ice core data for determining changes in sea ice extent over long time periods. Additionally sea salt aerosol, along with salty sea ice surfaces, is the host for the production of halogen compounds which seem to play a key role in the oxidation chemistry of the polar regions. It is therefore important to understand the sources of polar sea salt aerosol and therefore to be able to predict how they may vary with, and feedback to, climate.

It was recently proposed that the main source of this polar sea salt aerosol was the sublimation of salty blowing snow. The idea is that snow on sea ice has a significant salinity. When this salty snow is mobilised into blowing snow, sublimation from the (top of) the blowing snow layer will allow the formation of sea salt aerosol above the blowing snow layer, that can remain airborne after the blowing snow has ceased. First calculations suggested that this would provide a strong source of aerosol (greater than that from open ocean processes over an equivalent area). It was proposed that this would have a strong influence on polar halogen chemistry and a noticeable influence on halogens at lower latitudes. However, this was based on estimates of the relevant parameters as there were no data about aerosol production from this source, and almost no data about blowing snow over sea ice in general.

Here we propose to take advantage of a very rare opportunity to penetrate the Antarctic sea ice zone during winter, as we have been allocated spaces on an unusual winter cruise into the sea ice zone on the German icebreaker Polarstern. During this cruise, we will be able to confirm that the blowing snow sea ice source exists, and make measurements that will provide a soundly-based parameterisation of the source. This will be done by making measurements of the snow on sea ice, of the blowing snow itself, and of aerosol above the blowing snow, as well as before and after such episodes. Measurements will include salinity, chemistry (looking at the amount of bromine present in each medium), and for blowing snow and aerosol, the amounts and size distributions.

By combining our data with meteorological data, and by comparing them to satellite observations that have recently attempted to identify blowing snow episodes, we will be able to make estimates of the spatial and temporal distribution of sea salt aerosol from this source over the entire Antarctic sea ice zone. This will allow us to assess the importance of this source of sea salt (and of halogens) compared to others that have been proposed. We will then use existing models to assess how important such a source is to sea salt deposition in Antarctica, allowing us to determine how sea salt in ice cores is related to sea ice extent. This opens the possibility of turning a qualitative sea ice proxy into a quantitative one. Models will also be used to re-assess the importance of this source for halogen chemistry in the polar regions and globally.

In summary this proposal will provide the first targeted measurements of the parameters needed to assess the importance of blowing snow sublimation as a source of sea salt, and to quantify its most relevant impacts.

The main immediate beneficiaries of our research are academic ones. Our data will benefit process understanding and quantitative estimates in several fields. We have identified in particular:

* Atmospheric scientists studying sea salt aerosol globally, including its direct and indirect radiative effects
* Cryosphere/atmosphere scientists studying boundary layer processes, blowing snow, and its impact on mass balance
* Ice core scientists developing proxies from sea salt
* Atmospheric chemists studying the role of halogens in oxidation chemistry

Beyond this, because one major aim is to develop a quantitative sea ice extent proxy from sea salt in ice cores, we also identify a community of:

* Sea ice scientists, starting with the palaeo-sea ice community, and through their integrated findings, the sea ice modelling and prediction community.

The aim of learning about palaeo sea ice is to improve (confidence in) models of sea ice and therefore predictions. This in turn would lead to benefits to those who need to access the ice-covered areas of the polar regions, with obvious economic (shipping, resources, insurance) and geopolitical implications. However, our more fundamental science will not go this far down the impact pathway, and we consider our main responsibility in ensuring impact to be
* ensuring that our findings are incorporated into the latest UK Earth System models (via the Cambridge team's role in UKCA, and via scientific conference presentations and publication)
* ensuring that our construction of a sea salt ice core sea ice proxy is validated by the palaeo sea ice community and that the results of using the proxy are disseminated to the wider sea ice community; this will be done through activities around the IGBP-PAGES Sea Ice Proxy working Group, which PI Wolff co-chairs.

Finally, we as scientists have a role in making science exciting and accessible to the public and to students. Like many projects in the Antarctic, this project has considerable potential as an exemplar of how science is done and how exciting it can be. BAS and the PI's group have a strong record of public outreach to get exciting science across, and they will use their usual channels (which include many public lectures by the PI, visits to BAS by groups such as the Cambridge Programme for Industry, visits to schools and web material). Because the cruise is led by AWI, we will follow their lead on the overall outreach from the cruise (not available at the time of writing the proposal), but we will work this experiment and project into BAS's usual NC/KE-funded activities.